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authorDaniel Vetter <daniel.vetter@ffwll.ch>2015-04-15 19:34:43 +0200
committerDaniel Vetter <daniel.vetter@ffwll.ch>2015-05-04 10:57:00 +0200
commit99264a61dfcda41d86d0960cf2d4c0fc2758a773 (patch)
tree8af532ebfccbddc982fed5429e132ed0f4e7de64 /include/drm
parenta3c6d686443e912e33cebdf9cd80f94df3ded7b0 (diff)
drm/vblank: Fixup and document timestamp update/read barriers
This was a bit too much cargo-culted, so lets make it solid: - vblank->count doesn't need to be an atomic, writes are always done under the protection of dev->vblank_time_lock. Switch to an unsigned long instead and update comments. Note that atomic_read is just a normal read of a volatile variable, so no need to audit all the read-side access specifically. - The barriers for the vblank counter seqlock weren't complete: The read-side was missing the first barrier between the counter read and the timestamp read, it only had a barrier between the ts and the counter read. We need both. - Barriers weren't properly documented. Since barriers only work if you have them on boths sides of the transaction it's prudent to reference where the other side is. To avoid duplicating the write-side comment 3 times extract a little store_vblank() helper. In that helper also assert that we do indeed hold dev->vblank_time_lock, since in some cases the lock is acquired a few functions up in the callchain. Spotted while reviewing a patch from Chris Wilson to add a fastpath to the vblank_wait ioctl. v2: Add comment to better explain how store_vblank works, suggested by Chris. v3: Peter noticed that as-is the 2nd smp_wmb is redundant with the implicit barrier in the spin_unlock. But that can only be proven by auditing all callers and my point in extracting this little helper was to localize all the locking into just one place. Hence I think that additional optimization is too risky. Cc: Chris Wilson <chris@chris-wilson.co.uk> Cc: Mario Kleiner <mario.kleiner.de@gmail.com> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com> Cc: Michel Dänzer <michel@daenzer.net> Cc: Peter Hurley <peter@hurleysoftware.com> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> Reviewed-and-tested-by: Mario Kleiner <mario.kleiner.de@gmail.com> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Diffstat (limited to 'include/drm')
-rw-r--r--include/drm/drmP.h8
1 files changed, 6 insertions, 2 deletions
diff --git a/include/drm/drmP.h b/include/drm/drmP.h
index 62c40777c009..4c31a2cc5a33 100644
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -686,9 +686,13 @@ struct drm_pending_vblank_event {
struct drm_vblank_crtc {
struct drm_device *dev; /* pointer to the drm_device */
wait_queue_head_t queue; /**< VBLANK wait queue */
- struct timeval time[DRM_VBLANKTIME_RBSIZE]; /**< timestamp of current count */
struct timer_list disable_timer; /* delayed disable timer */
- atomic_t count; /**< number of VBLANK interrupts */
+
+ /* vblank counter, protected by dev->vblank_time_lock for writes */
+ unsigned long count;
+ /* vblank timestamps, protected by dev->vblank_time_lock for writes */
+ struct timeval time[DRM_VBLANKTIME_RBSIZE];
+
atomic_t refcount; /* number of users of vblank interruptsper crtc */
u32 last; /* protected by dev->vbl_lock, used */
/* for wraparound handling */